One highly efficient linear amplifying apparatus is a high frequency amplifying apparatus that uses a saturated amplifier employing Linear Amplification with Nonlinear Components (LINC). Conventionally, such amplifying apparatuses include a pair of amplifiers and a coupler, where signals output from the amplifiers are combined by the coupler and output. A hybrid coupler or a 3 dB coupler is used.
One conventional amplifying apparatus divides a high frequency signal into 2 signals identical in amplitude and differing in phase, amplifies the resulting signals, transmits the amplified signals by transmission paths having a length of λ/4, and combines the signals. In the amplifying apparatus, a λ/8 open stub is connected to one transmission path and a 3λ/8 open stub is connected to the other transmission path. In another amplifying apparatus, 2 transmission paths respectively having a filtering property are connected by a filter circuit positioned an equal distance from amplifiers that are respectively disposed upstream of the transmission paths. In yet another amplifying device, multiple capacitors and switching circuits for frequency matching are disposed at the secondary side of the output transformer of the amplifier and by switching between the switching circuits, a capacitor at the secondary side of the output transformer is selected. In addition, another apparatus is configured by pure inductive circuit elements and the apparatus combines 2 signals identical in frequency and differing in phase, and generates a signal that is proportional to the sum or difference of the 2 signals.
For examples of conventional technologies refer to Japanese Laid-Open Patent Publication Nos. 2008-135829; 2009-182397; 2009-130897; and H7-131278.
Japanese Laid-Open Patent
Nonetheless, with conventional amplifiers, when signals are combined by a coupler such as a hybrid or 3 dB coupler, a loss of 3 dB occurs. On the other hand, when a pair of amplifiers is coupled by a coupler without coupler isolation such that a suitable effect with respect to the respective loads of the amplifiers is achieved, efficiency can be maintained over a wider dynamic range as compared to a conventional amplifier.
FIG. 1 is a diagram of an amplifying apparatus employing a coupler without coupler isolation. The amplifying apparatus depicted in FIG. 1 includes transmission lines 4 and 5 of differing lengths, as a coupler without coupler isolation. The amplifying apparatus generates uses a signal generator 1 to generate from an input signal with a varying envelope, a pair of signals constant in amplitude and differing in phase, amplifies the signals output from the signal generator 1 by amplifiers 2 and 3, and then combines for output, the signals output from the amplifiers 2 and 3 by the transmission lines 4 and 5.
Nonetheless, with an amplifying apparatus such as that depicted in FIG. 1, when the pair of amplifiers is coupled to maintain efficiency over a wide dynamic range, the dynamic range of the amplitude of the output power of the coupler is significantly narrower than desired. In other words, the amplitude modulation of the signal input to the amplifying apparatus cannot be reproduced.
With respect to the output power of the amplifying apparatus, an imbalance of the amplitudes of the signals input to the pair of amplifiers can be considered as one cause of the inability to reproduce the amplitude modulation. Therefore, by independently adjusting the amplitudes of the signals input to the pair of amplifiers, the amplitudes could be balanced theoretically. However, for example, in an amplifying apparatus that, similar to an amplifying apparatus employing LINC, uses the amplifier in a saturated state to improve efficiency, even if the amplitude of the signal input to the amplifier is increased, the power out from the amplifier does not substantially change. On the other hand, if the amplitude of the signal input to the amplifier is decreased, the amplifier no longer operates in a saturated state and thus, efficiency deteriorates.